Field of the Invention
The present invention relates to a transfer head for a micro light-emitting diode (micro LED), which is configured to grip a micro LED and transfer the micro LED to a display substrate.
Description of the Related Art
Currently, the display market is still dominated by LCDs, but OLEDs are quickly replacing LCDs and emerging as mainstream products. In a current situation where display makers are rushing to participate in the OLED market, micro light-emitting diode (hereinafter, referred to as micro LED) displays have emerged as another next generation display. Liquid crystal and organic materials are the core materials of LCDs and OLEDs, respectively, whereas the micro LED display uses 1 μm to 100 μm of an LED chip itself as light emitting material.
Since the term “micro LED” emerged in a patent “MICRO-LED ARRAYS WITH ENHANCED LIGHT EXTRACTION” in 1999 (Korean Patent No. 10-0731673, hereinafter referred to as ‘Related Art 1’) disclosed by Cree Inc., related research papers based thereon were subsequently published. In order to apply the micro LED to a display, it is necessary to develop a customized microchip based on a flexible material and/or flexible device using a micro LED device, and techniques of transferring the micrometer-sized LED chip and mounting the LED chip on a display pixel electrode are required.
Particularly, with regard to the transfer of the micro LED device to the display substrate, as the LED size is reduced to 1 μm to 100 μm, it is impossible to use a conventional pick-and-place machine, and a technology of a transfer head for higher precision is required. With respect to such a technology of a transfer head, several structures have been proposed as described below, but each of the proposed techniques has some problems.
Luxvue Technology Corp., USA, proposed a method of transferring a micro LED using an electrostatic head (Korean Patent Application Publication No. 10-2014-0112486, hereinafter referred to as ‘Related Art 2’). A transfer principle of the Related Art 2 is that a voltage is applied to a head portion made of a silicone material so that the head portion comes into close contact with a micro LED due to electrification. However, this method may cause damage to micro LEDs due to electrification caused by the voltage applied to the head portion during induction of static electricity.
X-Celeprint Limited, USA, proposed a method of using an elastic polymer material as a transfer head and transferring a micro LED positioned to a wafer to a desired substrate (Korean Patent Application Publication No. 10-2017-0019415, hereinafter referred to as ‘Related Art 3’). According to the Related Art 3, there is no problem in LED damage as compared with the above-mentioned electrostatic head case. An adhesive force of the elastic transfer head is required to be higher than that of a target substrate in the transfer process to transfer a micro LED stably, and an additional process for forming an electrode is required. In addition, maintaining an adhesive force of the elastic polymer material is an important factor.
Korea Photonics Technology Institute proposed a method of transferring a micro LED using a ciliary adhesive-structured head (Korean Patent No. 10-1754528, hereinafter referred to as ‘Related Art 4’). However, in the Related Art 4, it is difficult to manufacture a ciliary adhesive structure.
Korea Institute of Machinery and Materials has proposed a method of transferring a micro LED using a roller coated with an adhesive (Korean Patent No. 10-1757404, hereinafter referred to as ‘Related Art 5’). However, the Related Art 5 has a problem in that continuous use of the adhesive is required, and the micro LED may be damaged when pressed with the roller.
Samsung Display Co., Ltd proposed a method of transferring a micro-LED to an array substrate according to electrostatic induction by applying a negative voltage to first and second electrodes of the array substrate in a state where the array substrate is immersed in a solution (Korean Patent Application Publication No. 10-2017-0026959, hereinafter referred to as ‘Related Art 6’). However, the Related Art 6 has a problem in that a solution is required since the micro LED is immersed in the solution to transfer to the array substrate, and a drying process is required.
LG Electronics Inc. proposed a method in which a head holder is disposed between multiple pick-up heads and a substrate and a shape of the head holder is deformed by movement of the multiple pick-up heads such that the multiple pick-up heads are allowed to move freely (Korean Patent Application Publication No. 10-2017-0024906, hereinafter referred to as ‘Related Art 7’). However, the Related Art 7 has a problem in that a process of applying a bonding material to the pick-up heads is required because the bonding material having an adhesive force is required to be applied to bonding surfaces of the multiple pick-up heads to transfer the micro LED.
In addition to the problems of the above-described related arts, the related arts have a problem in that, when transferring the micro LED from a growth substrate to a display substrate, a grip surface of a transfer head comes into contact with an upper surface of the micro LED.
In detail, the grip surface of the transfer head comes into direct contact with the micro LED during lowering of the transfer head, a position of the micro LED formed on the growth substrate is mislaid, and a positional error may occur in the display substrate when mounted on the display substrate. Furthermore, since the micro LED has a very small size, the micro LED may be damaged when coming into contact with the grip surface of the transfer head.
In order to solve the problems of the related arts described above, it is necessary to solve the above-mentioned problems while adopting the basic principles adopted in the related arts. However, there is a limit to solving the problems because such problems are derived from the basic principles adopted in the related arts. Therefore, applicants of the present invention have not only solved the problems of the related arts but also proposed an advanced method which has not been considered in the related arts.